1. Field of the Invention
This invention relates to technology for generating a programming current supplied for setting the light emission level of a pixel circuit in a luminescent device.
2. Description of the Related Art
In recent years, electro-optical devices have been developed using organic electroluminescent devices. A backlight is unneeded for organic electroluminescent devices as they are self-luminescent, so it is expected that they will be used to achieve display devices with low power consumption, a wide viewing angle and a high contrast ratio. In the present specification, an “electro-optical device” refers to a device for converting electrical signals to light. The most common form of an electro-optical device is a display device for converting electrical signals representing images to light representing images.
In an active matrix driven electro-optical device using organic electroluminescent devices, a pixel circuit is provided to adjust the light emission level or luminescent scale of each organic electroluminescent device. The light emission level in each pixel circuit is set by supplying a voltage or current value to the pixel circuit corresponding to the light emission level. The method of setting a light emission level using voltage is called a voltage programming method, and that for setting a light emission level using a current value is called a current programming method. Herein, the term “programming” is used to mean “setting the light emission level”. In the current programming method, the current used when programming a pixel circuit is called the “programming current”. In a current programming type electro-optical device, a current generation circuit is used to generate a programming current having an accurate current value corresponding to the light emission level and supplying it to each pixel.
A programming current value corresponding to the light emission level, however, depends on the structure of the pixel circuit. The structure of pixel circuits often differs somewhat according to the design of the electro-optical device. Thus, there has been desired a current generation circuit whose range of output current values (programming current values) is easy to set according to the actual structure of the pixel circuit.